Optical positions for a sample of ICRF sources

The aim of this paper is to present a large, coherent set of astrometric op tical positions for ICRF sources in the Hipparcos reference frame. For this , we present positions for 315 ICRF sources measured in Digitized Sky Surve y (DSS) images, with positions referred to the ACT, Hipparcos, and Tycho ca talogs. The reductions were made relative to Hubble Space Telescope guide s tars, which were previously corrected toward those same catalogs. The reduc tions errors are at 45 mas and even smaller for southern sources. The compa rison between optical with ICRF radio positions shows that neither the resi duals nor the internal errors exhibit any trend with magnitudes or with loc ation on the DSS plates.